EVOLVING METAL RATIOS IN THE LONG-LIVED SULPHURETS PORPHYRY AU-CU-MO DISTRICT: A LINK BETWEEN METAL ENDOWMENT IN PORPHYRY COPPER DEPOSITS AND DEEP CRUSTAL PROCESSES

Porphyry copper deposits are the source of significant proportions of the world’s copper, gold, and molybdenum. However, the controls governing the total metal endowment and the total Au/Cu and Mo/Cu ratios for a given porphyry copper deposit are still unclear. New Re-Os molybdenite geochronological data was obtained for the Early Jurassic Sulphurets porphyry Au-Cu-Mo district, located in British Columbia, Canada. The Sulphurets district porphyry ores are unusually gold-rich and contain one of the largest global porphyry gold endowments, totaling 2,748 t Au (total Measured & Indicated resources). The geochronology data indicate that mineralization events occurred in periodic pulses, resulting in the formation of four separate deposits over the course of 11 ± 3.5 m.y. – an exceptionally long lifespan for a porphyry copper district. This protracted persistence and the punctuated nature of mineralization highlights the complexity and potentially prolonged nature of crustal-scale magmatism in porphyry copper deposit development. Furthermore, the district deposits demonstrate progressive increases in their endowment of Au and Mo as well as in their proportions of Au/Cu and Mo/Cu over time. The four known deposits (Kerr, ~201-203 Ma), Sulphurets and Iron Cap (~195-194 Ma), and Mitchell (~192-190 Ma) record an evolution of metals from early low Au/Cu and Mo/Cu to late high Au/Cu and Mo/Cu that parallels magmatic compositional evolution from low Ce/V to high Ce/V. The giant Au- and Mo-rich giant Mitchell deposit is proposed to be the product of protracted magmatic compositional evolution. The ~11 m.y. district lifespan suggests that incompatible metal and volatile build up occurred in late differentiated magmas in the mid- or deep-crust, as shallow crustal magmatic systems could not be sustained over such periods. In these deep environments, periodic mafic magma input, crystal fractionation, and crustal assimilation processes produced metal evolution. This case study presents a robust case that systematic metallogenetic evolution over time in a porphyry copper district can ultimately be driven by deep crustal magmatic processes.